1. Field of the Invention
The present invention relates in general to ultrasonic sensors for use in detecting obstacles or the like by transmission and reception of ultrasonic waves and, more particularly, to improvements in ultrasonic sensors having a piezoelectric element and temperature compensation capacitor contained in a casing.
2. Description of the Prior Art
Conventionally, known ultrasonic sensors have been adapted for use as obstacle detection sensors in land vehicles, for example. FIG. 12 is a cross-sectional diagram for explanation of one example of the ultrasonic sensors of this type.
As shown in FIG. 12, an ultrasonic sensor 61 includes a cylindrical casing 62 made of a desired metal having one closed end. A closed end surface 62a of the casing 62 is adapted for transmission and reception of ultrasonic waves.
A piezoelectric vibration element 63 is secured inside the end surface 62a of casing 62. The piezoelectric vibration element 63 includes a piezoelectric ceramic plate which has a structure for allowing electrodes (not shown) to be disposed on the two principal surfaces of the piezoelectric ceramic plate comprised of PZT-based piezoelectric ceramics.
A sound-absorbing member 64 made of a desired polyester felt material is disposed near the piezoelectric vibration element 63. Also within the casing 62, a temperature compensation capacitor 65 is disposed above the sound-absorbing member 64. The temperature compensation capacitor 65 has a structure which allows for electrodes to be formed on the both principal surfaces of the temperature compensation capacitor 65. One principal-surface electrode of the temperature compensation capacitor 65 is electrically connected by a lead wire 66a to the piezoelectric vibration element 63. The remaining principal-surface electrode of temperature compensation capacitor 65 is electrically connected via a lead wire 66b to the casing 62. Note that the casing 62 is electrically coupled to the other principal surface (a lower surface as visible in the drawing) of the piezoelectric vibration element 63.
A couple of take-out or "external" lead wires 67a, 67b are connected to the electrodes on the both principal surfaces of temperature compensation capacitor 65, respectively. The external lead wires 67a, 67b are electrically coupled to a connector 68 provided outside of the casing.
The inside space of the casing 62 is filled with sound-absorbing silicon adhesive 69 for purposes of elimination of unwanted reflection of ultrasonic waves. Also, a sound-absorbing member 70 is arranged so as to cover the open end side of the casing 62. The sound-absorbing member 70 may be made of expanded polyurethane.
In the ultrasonic sensor 61, the temperature compensation capacitor 65 is arranged and constructed in consideration of the fact that the electrostatic capacitance of the piezoelectric vibration element 63 can significantly vary with a change in temperature resulting in the resonance characteristics being varied or deviated.
However, elimination of the temperature compensation capacitor 65 would reduce workability of the bonding connections made via soldering of lead wires 66a, 66b, 67a, 67b during assembly of the ultrasonic sensor 61.
More specifically, during assembly of the ultrasonic sensor 61, it is necessary that the bonding connections by use of the lead wires 66a, 66b be completed prior to injection or packing of the silicon resin 69 in the casing 62. Unfortunately, since the temperature compensation capacitor 65 is structured and arranged so that the electrodes are disposed on the two principal surfaces of the temperature compensation capacitor 65, it is necessary to connect each individual one of such lead wires 66a, 66b by soldering techniques to a corresponding one of the electrodes on respective principal surfaces of the temperature compensation capacitor 65 individually while simultaneously forcing the temperature compensation capacitor 65 to be kept stationary at a certain position shown in FIG. 12, which increases the complexity and difficulty of the wire-bonding process.
In addition, it is required during injection of the silicon resin 69 that workers verify by visual inspection whether the lead wires 66a, 66b are reliably and securely in contact with the temperature compensation capacitor 65 and make sure that the lead wires 66a, 66b have not accidentally become detached from the temperature compensation capacitor 65. Because this structure has the lead wires 66a, 66b coupled to both principal surfaces of the temperature compensation capacitor 65, it is necessary to visually check a respective one of the two principal surfaces of temperature compensation capacitor 65 by time-consuming visual inspection methods which are subject to human error and errors in judgment.
Further, with regard to the bonding connection of lead wires 67a, 67b, it has been necessary for workers to verify--prior to hardening of the sound-absorbing material 70--that the lead wires 67a, 67b are reliably coupled with the temperature compensation capacitor 65. It has also been necessary that workers inspect a respective one of the two principal surfaces of the temperature compensation capacitor 65 due to the fact that such lead wires 67a, 67b are to be connected to the electrodes on both principal surfaces of the temperature compensation capacitor 65, respectively.
Furthermore, since the temperature compensation capacitor 65 has a structure for eliminating the electrostatic capacitance from the electrodes located on both principal surfaces of the temperature compensation capacitor 65, the resulting overall size of the device remains relatively large. Accordingly, distortion can take place upon application of thermal shocks from outside of the device. Another problem experienced in the prior art devices is that during injection of silicon resin 69, gas bubbles or air voids can appear within the casing 62 due to ultrasonic wave reflection because of the relatively large size of the temperature compensation capacitor 65 and also because of an increased area required for contact between the silicon resin 69 and the temperature compensation capacitor 65.
A further problem is that ultrasonic-wave reflection caused by the temperature compensation capacitor 65 per se can be present above negligible levels because of the fact that the temperature compensation capacitor 65 must have a relatively large size.
In addition, arranging the temperature compensation capacitor 65 in a specific orientation in which the principal surfaces of the temperature compensation capacitor 65 are at right angles to the piezoelectric vibration element 63 results in a noticeable increase in distance from the upper end of the temperature compensation capacitor 65 to the piezoelectric vibration element 63. For this reason, the temperature sensitivity or tracking ability of the capacitor 65 with respect to temperature variations of vibration element 63 is insufficient which makes it impossible to achieve sufficient temperature compensation functions.